# A factory uses a radioactive source to monitor the thickness of aluminium sheets as they are produced on a production line. The source is placed on one side of the sheet and a detector is placed on the other side. Describe how this thickness monitoring system works, including why a beta emitter with a long half-life is used rather than an alpha or gamma emitter.

> Pearson Edexcel International GCSE Physics (4PH1) — 7.2 Radioactivity · Describe · 4 marks

> In industrial thickness monitoring, the count rate recorded by the detector changes if the thickness of the sheet changes. The source chosen must have a suitable type of radiation and a long half-life.

## Mark scheme (4 marks)

1. Beta particles are used because they are stopped by / their count rate is reduced by changes in the thickness of aluminium, whereas alpha particles would be stopped entirely by the sheet and gamma rays would pass straight through with little change in count rate.
2. If the sheet becomes too thick, fewer beta particles reach the detector, so the count rate drops.
3. If the sheet becomes too thin, more beta particles reach the detector, so the count rate rises.
4. A long half-life is required so that the activity / count rate does not decrease significantly over time, ensuring readings remain reliable and do not need to be attributed to decay of the source.

## Key terms

- [half-life](https://www.gradenine.co.uk/glossary/half-life)

## Related

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-factory-uses-a-radioactive-source-14a079a5) · Published by Druglandscape Ltd.